The present invention relates generally to a serial printer wherein a print head supported by a carriage effects a printing operation while the print head is moved along a line of printing, and more particularly to a technique for improved accuracy of controlling the printing points or positions at which the print head is activated.
Means for controlling the printing actions of the print head of a serial printer are disclosed in Japanese Utility Model Application laid open for opposition purpose on June 23, 1977 under Publication No. 52-27635, and in U.S. Pat. No. 4,213,714. In a printer disclosed in the former document, the print head is controlled according to printing commands which are generated at a predetermined frequency, independently of an operation of a drive motor to move the carriage. In a printer disclosed in the latter document, the carriage drive motor has an encoder, and the printing actions of the print head are controlled according to output signals generated by the encoder.
In the printer of the laid-open Publication No. 52-27635, however, the operating or printing positions or points of the print head do not have a direct relation with the incremental position of the carriage. According to this arrangement, operating errors of a timer used and/or variations in signal processing time accumulate during repeated control cycles for the successive printing actions, whereby the print head tends to be activated when the carriage (i.e., the print head) is not located at the appropriate positions along the print line. Thus, the printer is likely to suffer from undesirable deviation of the actual printing positions from the nominal printing positions.
The printer disclosed in the above-identified U.S. Patent does not suffer from such a problem, but requires an expensive rotary encoder. Described more specifically, this printer uses a rotary encoder in the form of a disc which is rotated with a carriage drive motor. The encoder disc has a multiplicity of radially extending slits. As the disc is rotated, passage of the slits is detected by a transducer, which produces pulse signals corresponding to the detected slits. A printing command is applied to the print head, when the number of the produced pulse signals reaches a predetermined value. By changing this value, the width of each character to be printed can be varied in a plurality of steps. This arrangement therefore requires the slits of the disc to be arranged at an extremely small angular interval. In other words, the encoder disc must be formed with a very large number of slits, which inevitably pushes up the cost of manufacture.